Conventional glass fiber matrix composites have an excellent strength-to-weight ratio and are resistant to heat at moderate temperature levels; however, they are unsuited for applications where they may be exposed to flame or high temperature, because the conventional composites are not flame resistant, and because both the glass fibers and the resin lose structural integrity at high temperatures. In addition, the resins commonly used in conventional composites emit toxic gases as they are heated. For this reason, conventional glass fiber composites are unsuitable for constructing fire walls, jet engine duct liners, and other structures that may likely be exposed to flame or high temperatures.
Temperature resistant fibers, such as aluminoborosilicate fibers (sold by Minnesota Mining and Manufacturing Company under the trademark NEXTEL), are available for use in manufacturing temperature resistant composites. A bonding problem exists, however, between the NEXTEL fibers and the high temperature resins, which must be used in temperature resistant composites. Prior to use in the composite, the NEXTEL is heated to remove sizing applied during its manufacture, and then is fired at a temperature in excess of 900.degree. C., increasing the fibers' tensile modulus significantly. The heat treatment removes the hydroxyl groups (normally found on the surface of the fibers) that are important for bonding an organic resin to the fibers. If the resin is applied to these hydroxyl-deficient fibers, the resulting composites are low in strength and flexural modulus, particularly at elevated temperatures. If the resin were more securely bound to the fibers, the strength and modulus could be substantially improved. Other types of ceramic fibers also exhibit the lack of bonding sites (i.e., hydroxyl groups) necessary to prepare high temperature composites of acceptable strength.
While several solutions are plausible to overcome the lack of binding sites, none are particularly desirable. For example, others have sought to etch NEXTEL with organic or inorganic caustics or with inorganic acids, such as HF or H.sub.2 NO.sub.3, and to use these etched fibers with or without coupling agents in composites--all without significant improvement over use of the untreated fibers alone.
Ceramic fiber composites may serve as lightweight replacements for metal structures, if they can be fabricated to have acceptable strength. The present invention proposes one way of improving the bonding of the resin to the ceramic fibers which overcomes the bonding site deficiency previously encountered when fabricating these types of composites.
Accordingly, this invention provides an improved bond between a ceramic fiber and an organic resin applied to the fiber to form a composite. This advantage and others will be apparent from the attached drawings and the description of the preferred embodiments that follows.